It is well understood that the fuel efficiency and aerodynamic performance of an aircraft is dependent on wingspan. It is therefore important to ensure that all aircraft are manufactured with their optimum wingspan.
For passenger aircraft, a limit on wingspan is set by the physical characteristics of gates at airport terminals. Such gates have historically accommodated aircraft with a maximum wingspan of thirty-six meters, meaning that aircraft with wingspans exceeding the thirty-six meter limit are generally unable to taxi to the gate. This makes larger wingspan aircraft much less practical than the smaller aircraft currently in use.
Modification of airport terminals to accommodate larger wingspan aircraft would be both expensive and disruptive. It would also necessarily lead to a corresponding reduction in the number of aircraft which could be accommodated simultaneously at the terminal, and therefore a reduction in overall airport capacity. As such, another solution must be found in order to improve the practicality of larger wingspan aircraft.
One proposal has been to provide large wingspan aircraft with a “piano-hinge” at the tip end of the wing, allowing the wing tip to be folded upwards from a horizontal configuration to a vertical configuration upon landing. This is shown in FIG. 1. In the vertical configuration, the wingspan of the aircraft is reduced so that the aircraft can access restricted size terminal gates.
However, it will be appreciated that the proposed piano-type hinge requires a significant amount of torque to be exerted in order to lift the wing tip out of the horizontal configuration. Providing such a large torque requires the wing to be equipped with a high-torque lifting means. High-torque lifting means of this type are heavy, and therefore their inclusion undesirably adds significant weight to both the wing and the aircraft. Furthermore, because the torque required to lift the wing tip rapidly reduces as the tip moves towards the vertical configuration, the high-torque capability of the lifting means is wasted for all but the initial phase of the lift. This proposal is therefore both extremely inefficient and heavy.
Whilst the vertical configuration of the wing described above may provide the potential for a large wingspan aircraft to access a restricted size airport gate, it is obviously crucial that the wing does not enter the vertical configuration when the aircraft is in flight. Previous folding wing designs have addressed this issue by employing a simple “shot-bolt” clamp to lock the wing tip in the horizontal configuration. It will be appreciated that a small amount of clearance is required in such “shot bolt” clamps in order to allow the shot bolt to slide freely when the clamp is to be engaged/disengaged. This clearance allows a small amount of play within the clamp which, although relatively small in the clamp itself, can translate to a relatively large movement at the tip of the wing due to the distance between the tip and the clamp. This can cause the wing tip to flutter during flight, which is extremely undesirable.
The natural vibration of the wing during flight causes the components of the clamps to wear relatively quickly, especially if the wing tip has begun to flutter. This increases the amount of play within the clamp. As the wear becomes significant, large and potentially dangerous wing tip movements are observed during flight meaning that, in order to maintain aircraft safety, it is necessary to replace the clamp on a frequent basis.